SMA (Shape Memory Alloy)

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Scarr

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Hi,

This is sort of a electronics question, I am playing with SMA and intend to create a circuit to control a piece of SMA but before I can do this I need to train the SMA to the shape I want , apparently this is done by holding in shape and heating to 500c for 5mins then dump in water, anyone any sugestions on how to attain 500c using household / DIY equipment in a nice even enviroment?

I did have a thought about iduction heating or heating it like a light bulb filiment but thought I would ask here for peoples comments before going off at a tangent. e.g. Induction heating is far too expensive and dead short so not a good idea because....


P.S. SMA = Nitinol = Flexinol and a few other names.
 
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Just place it in a regular toaster oven on a metal tray. THe contact with the tray will ensure an even temperature despite air temperature fluctuations and I doubt the metal is sensitive enough to overheating to warrant any fancier method (but I'd check just to be sure).
 
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Will this get hot enough?

dknguyen,

What sort of temps does a toaster oven create? didn't think it was that high as I know my main oven only gets to 280c (as it states on dial)
 
500C is indeed toasty but, as you observed, no toaster oven is likely to go that high.

How big and what is the resistance of the piece of SMA you have? It might work just to pass a current through it and heat it to the desired temperature. Or perhaps concentrated sunlight using a magnifying glass or parabolic reflector would work.

The problem is, how do you determine when it's at 500C?
 
I guessed that

dknguyen, I guessed as much but thought it prudent to ensure I had not missed a trick

Thx anyway
 
A place that does stained glass should have a kiln. Some stained glass people melt different color glass together to make jewelry. You might be able to rent some time there if the thought of putting something other than glass into their oven doesn't scare them. I don't know if glass kilns get hot enough though.

On a side note, I think putting it somewhere in the vicinity of the girl in dknguyen's pix might get it close.
 
Scarr said:
dknguyen, I guessed as much but thought it prudent to ensure I had not missed a trick

Thx anyway
Click to expand...

You could always bypass the relay in the oven (**fire hazard** **shock hazard**) and then monitor it manually. We did this for our senior year project to retrofit the oven with electronic temperature controls.
 
A complex way to get such temperatures is with a salt bath. Just find a salt that melts at the right temperature, heat in an appropriate vessel until it just melts, then add your SMA wire.

A much simpler way is to get a wax indicating marker (crayon) for the temperature you need. Mark the SMA, then heat with a soft flame or stove burner until the wax just melts. The change is obvious.

Temperature indicating markers (crayons) are used widely for annealing and heat treating metals. In the US, welding supply places usually have them. A supplier for knife making would also probably have them. The McMaster-Carr part number is 3261K91 ($10.12 USD).

John
 

A stove! WHy didn't I think of that? lol
 
Blow toruch / Stove

I think that any method like torch / gas stove will not work as the wire needs to be 500C for 15mins and using naked flame will result in hot / cold spots, unless the flame(s) can be held far enough away to get a even temp but then it's all a questions of will it make the temp if it's far away.

I intend to get a IR temp meter so measuring it won't be a problem, what about PWM with with mosfet etc. they recomend this was to make the wire jump back to it's memory shape (70C) or would I strugle to get these temps?

Thx
 
I do not know what temperature a "self-cleaning" oven goes to. It's more than 300°C. Perhaps one place to start is simply to measure what it is. That can be set for any time you want.

Scarr said:
apparently this is done by holding in shape and heating to 500c for 5mins
Click to expand...

Is it 15 minutes or 5 minutes? **broken link removed** source gives it at 2 to 5 minutes, depending on thermal mass. I wire is at the low end of that.

A flame, open burner (gas or electric), etc. can all be done for 5 minutes quite easily. For 15 minutes, you may be stuck with a salt bath or a heat treating oven (assuming the self-clean doesn't suit it).

A lot of small shops used to do heat treating, but it seems most now send everything out. Thus, there can be some good bargains out there. I got mine for free with a hand grinder I bought.

If you are near a university, the physical sciences departments may have high-temp ovens too. Try chemistry, physics. metallurgy, materials (too obvious ).

John
 
Ceramics once was a very popular hobby, and this requires a kiln (very hot oven). You may be able to locate someone who still does it.
 
You'll want to use a K type thermocouple instead of an IR gun due to emissivity issues.

Fluke makes an IR thermometer model that has three different emissivity settings as well as a K-type thermocouple input. You'll need to buy an additional thermocouple to the one they include, as the one they include is only good up to 100C. Other K-type thermocouples are good up to 1000C.

I don't think hot spots will be a problem since it's metal and will conduct heat fantastically. Use multiple candles if it's a large/long piece of nitinol.

Ceramic firing kilns are also a really good idea. Any pottery place will have them. Our middle school art program even had one.
 
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It sounds like annealing the SMA may become a difficult sub-problem that will prevent getting on with the goal.

Annealing (and heat treating) metals can be done with expensive equipment and should be done that way for production and reproducibility.

In the field, learning to use a soft flame can do the same. For example, to anneal copper, heat it until you see the right colors, then cool slowly. Some people use a reducing flame (i.e., fuel rich)and look for the appearance of soot on the metal. Heat treatable steels can be done the same way, particularly 4130, O-1, and W-1. You must get to or exceed the annealing temperature. That's where wax indicators or color guides come in handy. The important thing is to avoid fast quenching/cooling that can lead to hardening.

Good luck. John
 
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